1. Field of the Invention
The present invention relates to an aqueous dispersion of a peelable coating composition which is intended to be used for temporary protection of articles and is effective in temporary storage of articles during distribution, where articles may be temporarily stored in the open air or exposed to the open air during distribution.
2. Description of the Related Art
In more and more cases, articles such as automobiles and other vehicles, machine parts, metallic household articles and other ferrous and non-ferrous articles, wooden articles, glass articles, rubber articles, and coated rubber articles (hereinafter collectively referred to as xe2x80x9carticlesxe2x80x9d) reach the hands of consumers after being transported from the manufacturer and stored temporarily in the open air. In such cases, during the period before these articles are in hands of consumers (generally this period is 2-12 months in duration), the surfaces of these goods are prone to becoming scratched, stained, discolored, or contaminated due to, for example, deposits such as sandy dust, iron powder, salts, alkalis, acids, soot and smoke, dead bodies or body fluids of insects, and excrement of birds and insects; sunlight; and wind and rain. Moreover, the material may come to have marks due to physical forces.
These deposits, etc. must be removed as soon as possible, as they may reduce the value of goods. However, removal of deposits requires elaborate work, which is accompanied by additional costs.
Thus, in order to temporarily protect surfaces of an article from being damaged by the aforementioned deposits, sunlight, elements, or scratches, and to eliminate the elaborate work and accompanying costs, there have been proposed a number of strippable coating compositions which areapplied to surfaces of an article when the article is shipped, and which, after a predetermined period of custody is over, can be easily stripped off.
For example, two types of strippable coating compositions are used for outer panels of automobiles: one is a so-called oily wax type in which paraffin wax is dispersed in a petroleum-based solvent, and the other is a type in which a mixture of calcium carbonate powder in paraffin wax is dispersed in a petroleum-based solvent.
These strippable coating compositions call for use of a petroleum-based solvent for removal, and in addition, they may raise the fear of environmental problems such as air pollution. Compositions of a type in which an alkali-soluble resin is the primary component are also used, and these require an alkali detergent for removal, and therefore involve the problem of environmental pollution such as water contamination.
Acccordingly, in order to eliminate the aforementioned problems, there have been proposed emulsion-type coating compositions which form a coating film capable of being peeled off by hand after they have carried out their function of protection (see, for example, Japanese Patent Publication (kokoku) Nos. 50-39685 and 44-29583).
However, the above-described conventional coating compositions of the emulsion type have the following drawbacks.
1) The coating compositions disclosed in Japanese Patent Publication (kokoku) No. 50-39685 are made from a single starting material. Therefore, their peeling property is not satisfactory under environmental conditions in which they are usually used. For example, they cannot be satisfactorily peeled off in a wide temperature range from low temperatures around 5xc2x0 C. to high temperatures around 40xc2x0 C. at low temperatures the formed film breaks during the peeling operation, whereas at high temperatures it stretches and thus is not quickly peeled off.
2) Japanese Patent Publication No. 44-29593 discloses a coating composition obtained through emulsion polymerization in water of acrylonitrile and an acrylic acid ester of an lower alkyl.
Aqueous dispersions of an acrylic resin which contains acrylonitrile as an essential component raise a problem that harmful nitrogen oxides are produced during incineration of waste films that have been peeled and cause environmental pollution.
In the meantime, regarding peelable coating compositions for use with automobile outer panels, there exists need for compositions capable of forming a protective film that exerts contrasting functions simultaneously in terms of adhesion; i.e., proper adhesiveness and holding power that does not permit release of the film with ease during water-resistance tests or that never permits release during transportation of vehicles under protection by the film, and ease in peeling of the film with fingers after the period of protection by the film is over.
To solve the above-described problems and meet the above-stated need, the present inventors have carried out careful studies on dispersion compositions that do not contain a nitrogen-containing vinyl monomer, and have found that an aqueous dispersion of a peelable coating composition having excellent peelability can be obtained by forming an e emulsion having the following characteristics: 1) the emulsion is an acrylic copolymer emulsion which is a mixture of two acrylic copolymer emulsions, wherein the acrylic copolymers have different predetermined glass transition temperatures, 2) the amount of ethylenically unsaturated monomers having acid value contained in the mixture is not higher than a predetermined level, and 3) the emulsion does not make use of a nitrogen-containing monomer. The present invention has been accomplished based on this finding.
Moreover, the present inventors have found that, in addition to the above-mentioned features 1) through 3), when 4) polymerization is performed under conditions in which at least one of the emulsions contains a reactive surfactant and 5) the particle size of the emulsions is controlled to fall within a predetermined range, there can be obtained an aqueous dispersion of a peelable coating composition having further improved water resistance and excellent adhesiveness, leading to completion of the present invention.
Accordingly, a first aqueous dispersion of a peelable coating composition of the present invention is characterized by comprising
(a) a mixture containing 5-40% by weight of an emulsion of an acrylic copolymer having a glass transition temperature (hereinafter referred to as xe2x80x9cTgxe2x80x9d) of not lower than 40xc2x0 C., and 95-60% by weight of an emulsion of another acrylic copolymer having a Tg ranging from xe2x88x9220xc2x0 C. through 5xc2x0 C.; wherein
(b) the acrylic copolymer emulsion mixture (a) contains ethylenically unsaturated vinyl monomers having acid value in a total amount of less than 2.0% by weight; and
(c) the acrylic copolymer emulsion mixture (a) contains no nitrogen-containing vinyl monomers in the form of a monomer.
A second aqueous dispersion of a peelable coating composition of the present invention is characterized by, in addition to the aforementioned requirements (a), (b), and (c),
(d) at least one of the acrylic copolymer emulsions to be incorporated into the mixture is an acrylic copolymer which has been polymerized by use of a reactive surfactant, and
(e) the particle size of the acrylic copolymer emulsions is not more than 200 nm.
Next will be described modes for carrying out the present invention.
A first mode of the present invention is drawn to the first aqueous dispersion of a peelable coating composition characterized by an acrylic copolymer emulsion having the below-described features (a) through (c). Synergism of these features afford, as demonstrated in Examples hereinbelow, not only excellent physicochemical properties including water resistance, adhesive strength, protective power, and weatherability during the period of protection, but also excellent peelability in a wide temperature range from low temperatures (about 5xc2x0 C.) to high temperatures (about 40xc2x0 C.) after the protection period is over, and in addition, when the after-use waste film is incinerated, the film does not generate nitrogen oxides and thus eliminates adverse effects of environmental pollution, etc.
(a) The acrylic copolymer emulsion mixture of the present invention contains 5-40% by weight of an emulsion of an acrylic copolymer having a glass transition temperature of not lower than 40xc2x0 C. and 95-60% by weight of an emulsion of another acrylic copolymer having a Tg ranging from xe2x88x9220xc2x0 C. through 5xc2x0 C.
(b) The acrylic copolymer emulsion mixture (a) contains ethylenically unsaturated vinyl monomers having acid value in a total amount of less than 2.0% by weight.
(c) The acrylic copolymer emulsion mixture (a) contains no nitrogen source such as acrylonitrile in the form of a monomer.
A second mode of the present invention is drawn to the second aqueous dispersion of a peelable coating composition of the present invention characterized by, in addition to the aforementioned requirements (a), (b), and (c), the following features (d) and (e). synergism of all these features afford, as demonstrated in Examples hereinbelow, excellent physicochemical properties including water resistance, adhesive strength, and so on.
(d) At least one of the acrylic copolymer emulsions to be incorporated into the mixture is an acrylic copolymer which has been polymerized by use of a reactive surfactant.
(e) The particle size of the acrylic copolymer emulsions is not more than 200 nm.
Examples of the acrylic copolymer emulsion of the present invention include, but are not limited to, copolymer emulsions of one or more acrylic acid alkyl esters and/or methacrylic acid alkyl esters and one or more vinyl monomers which are capable of being polymerized with the acrylic alkyl esters and/or methacrylic alkyl esters, wherein the vinyl monomers include vinyl acetate, hydroxyl-group-containing vinyl monomers such as hydroxyethyl acrylate, hydroxyethyl methacrylate,. hydroxypropyl acrylate, hydroxypropyl methacrylate, and allyl alcohol, epoxy-group-containing vinyl monomers such as glycidyl acrylate and glycidyl methacrylate, and aromatic vinyl monomers such as styrene, methylstyrene, dimethylstyrene, ethylstyrene, butylstyrene, and benzylstyrene; and copolymer emulsions of a carboxyl-group-containing vinyl monomer such as acrylic acid, methacrylic acid, itaconic acid, citraconic acid, and crotonic acid and the aforementioned one or more vinyl monomers which are capable of being polymerized with the acrylic alkyl esters and/or methacrylic alkyl esters.
It should, however, be noted that nitrogen-containing monomers including acrylonitrile, aminoalkyl acrylates such as diethylaminoethyl acrylate and aminodimethyl acrylate, cyanoethyl acrylate, and isocyanate adducts are excluded from constituent monomers of the aforementioned acrylic copolymer emulsions.
With this exclusion, the waste coating film does not generate nitrogen oxides when incinerated, thus eliminating the fear of environmental pollution.
According to the present invention, two acrylic copolymer emulsions having different Tg ranges are mixed. Specifically, the Tg range of a first acrylic copolymer is equal to or greater than 40xc2x0 C., and that of a second acrylic copolymer is from xe2x88x9220xc2x0 C. to 5xc2x0 C.
The proportions of these two emulsions are preferably such that the acrylic copolymer emulsion having a Tg of not lower than 40xc2x0 C. is between 5% by weight and 40% by weight inclusive and the acrylic copolymer emulsion having a Tg from xe2x88x9220xc2x0 C. to 5xc2x0 C. is between 95% by weight and 60% by weight inclusive.
More preferably, the proportions of these two emulsions are such that the acrylic copolymer emulsion having a Tg of not lower than 40xc2x0 C. is between 10% by weight and 35% by weight inclusive and the acrylic copolymer emulsion having a Tg from xe2x88x9220xc2x0 C. to 5xc2x0 C. is between 90% by weight and 65% by weight inclusive.
Also, the proportion of the ethylenically unsaturated vinyl monomers having acid value in the acrylic copolymer emulsion mixture system is not more than 2.0% by weight, more preferably 0.2-0.8% by weight, with respect to the weight of the entirety of the mixture.
If ethylenically unsaturated vinyl monomers having acid value are contained in amounts in excess of 2.0% by weight, water resistance of the resultant coating film becomes poor, and in addition, adhesive strength increases to render the film excessively heat- and weather- resistant and therefore the film may not be easily peeled off, which is not preferred.
The ethylenically unsaturated vinyl monomers having acid value may bexe2x80x94among those which constitute the aforementioned acrylic copolymer emulsionxe2x80x94carboxy-containing vinyl monomers such as acrylic acid, methacrylic acid, itaconic acid, citraconicacid, and crotonic acid. How ever, the ethylenically unsaturated vinyl monomers having acid value are not limited only to these compounds.
In the present invention, examples of the reactive surfactants include xe2x80x9cAqualon RNxe2x80x9d series (tradename, Dai-ichi Kogyo Seiyaku K.K.) which are surfactants of the polyoxyethylene alkylphenyl ether type in which a radical-polymerizable propenyl group has been introduced into the hydrophobic group; anionic surfactants, xe2x80x9cAqualon HSxe2x80x9d series (tradename, Dai-ichi Kogyo Seiyaku K.K.) which are based on the sulfuric ester salts of xe2x80x9cAqualon RNxe2x80x9d; acrylic anion surfactants, xe2x80x9cEleminol RS30xe2x80x9d and xe2x80x9cEleminol JS-2xe2x80x9d (tradenames, both by Sanyo Chemical Industries, Ltd.) and xe2x80x9cAdeka Reasoapxe2x80x9d series (trade name, Asahi Denka Kogyo K.K.).
When reactive surfactants are added upon synthesis of acrylic copolymer emulsions, water resistance of the coating film is improved, film strength is enhanced, and peelability of the film is promoted.
The particle size of the acrylic copolymer emulsions is preferably not more than 200 nm, more preferably not more than 100 nm. When the particle size is in excess of 200 nm, a sufficient contact area cannot be obtained between the composition and the surface of the object to be coated, which leads to poor water resistance.
The aforementioned mixture may incorporate, as desired, small amounts of additives which may be useful in the manufacture of peelable-coating compositions. Such additives include plasticizers (e.g., phthalate esters and fatty acid esters), wax emulsions (including polyethylene-based emulsions and polypropylene-based emulsions), defoaming agents (e.g., mineral oils and silicone oils), Rheology modifier (including inorganic type and organic type), UV absorbents (e.g., benzotriazoles and benzophenones), pH modifiers (e.g., organic alkalis and ethanolamines), preservatives (e.g., benzoisothiazolines and triazines), coalescing agents (e.g., alkyl ethers of mono- or di-ethylene glycol and alkyl ethers of mono- or di-propylene glycol), anti-freezing agents (e.g., polyhydric alcohols), peelability-improving agents (e.g., silicone emulsions and alkylphosphoric esters), drying accelerators (e.g., lower alcohols such as ethyl alcohol).
The peelable coating composition of the present invention is intended to be applied onto surfaces of articles such as automobiles and other vehicles, machine parts, metallic household articles and other ferrous and non-ferrous articles, wooden articles, glass articles, rubber articles, and coated rubber articles, to thereby protect the surfaces of the articles in the open air or indoors. Needless to say, the composition of the present invention may be used for protecting surfaces of objects other than the above articles.
Specifically, the aqueous dispersion of peelable coating composition of the present invention is advantageously used, for example, for the following purposes.
1) Prevention of stains and scratches, or degradation due to UV rays, etc. of panel coatings and resin portions of automobiles.
2) Prevention of rust stains of machine parts.
3) Prevention of stains of edible oil and dirt from hands, when applied to ventilation fans and kitchen devices and utensils,
4) Prevention of stains and scratches of aluminum fences.
5) Prevention of stains and scratches of indoor floors.
6) Protection of the wall or floor materials in a paint booth against splashing of paints.
7) Prevention of scratches, or maintenance of good appearance when applied to skis made of an FRP resin.